The invention relates to making material that can withstand high temperatures in an oxidizing medium, in particular in the presence of air, steam, and more generally in the presence of any gaseous or liquid phase that contains oxygen or a compound of oxygen.
The invention relates in particular to making a refractory material part that is suitable for providing protection capable of withstanding high temperatures in an oxidizing medium. The invention also relates to providing protection against high temperatures in an oxidizing medium to thermostructural composite materials that are made at least in part out of carbon, with the fibers constituting the fiber reinforcement of such materials generally being carbon fibers, and it also being possible for the matrix densifying such materials to be made in part or in full out of carbon, or else out of a material other than carbon. The invention relates more particularly, but not exclusively, to carbon/carbon (C/C) thermostructural composite materials that are constituted by carbon fiber reinforcement densified by a carbon matrix.
Thermostructural composite materials are characterized by their mechanical properties that make them suitable for constituting structural parts, and by their ability to conserve those mechanical properties at high temperatures. Nevertheless, when they contain carbon, composite materials present the major drawback of oxidizing from 400° C. in air or in an oxidizing medium, and of losing their thermostructural properties in part.
For temperatures below 2000° C., there presently exist numerous anti-oxidation protective coatings for parts that are made at least in part out of carbon or graphite. The table below gives examples of protective coatings that can be used as a function of the maximum temperature of use under consideration.
MaximumutilizationtemperatureAnti-oxidation protection 700° C.B2O3 850° C.Zn2P2O71250° C.-1300° C.SiC + (barium aluminum borosilicate(SABB) + MoSi2) + (SABB + Y2O3)1500° C.Silicates of yttriumSiC + (magnesium aluminumborosilicate (SABM) + MoSi2)SiC + Al2O3 + mullite1600° C.SiC + Silicates of yttrium + SiO2C +SiC + Si3N41700° C.-1800° C.Silicon nitrideSilicon carbide
Nevertheless, above the temperatures specified in the above table, and a fortiori at above 2000° C., several phenomena can arise that are harmful to the effectiveness of the protection. Mention may be made in particular of problems of oxides presenting thermal and mechanical instability, poor protection against diffusion of oxygen, and separation between the coating and the substrate leading to oxidation along the interface between the carbon substrate that is to be protected and the protective coating.
No simple system satisfies all of those constraints. Multiphase systems have been envisaged for protecting thermostructural composite materials (e.g. C/C) at high temperatures, such as, in particular, hafnium di-boride (HfB2) or zirconium di-boride (ZrB2) as described in document U.S. Pat. No. 5,420,084, and they have been found to be good candidates for protective materials since they possess the following qualities in particular:                melting temperatures of about 3200° C.;        low specific gravity (6.09 and 10.5);        high hardness;        high electrical and thermal conductivity;        high resistance to thermal shock; and        good resistance to oxidation at high temperatures.        
In an oxidizing atmosphere, ZrB2 and HfB2 form a refractory oxide that is porous at a temperature higher than 2000° C. and a liquid phase B2O3 (melting temperature about 450° C.). Nevertheless, that liquid phase B2O3 evaporates almost completely when the temperature is higher than 1800° C. In order to lead to a less volatile liquid phase being formed, the refractory compound SiC (Td=2730° C.) has been added to ZrB2 and HfB2 so as to obtain a fluid borosilicate that is more stable at high temperature, while still possessing the ability to flow into the pores of the refractory oxide layer. By adding SiC to HfB2 and ZrB2, the oxidation of those compounds leads to a porous refractory skeleton made of HfO2 or ZrO2 that withstands high temperatures and that is coated on its surface in a viscous liquid phase constituted by SiO2, which has the property of reducing the quantity of oxygen diffusing through the oxide layer, and consequently of reducing the rate at which the protective material oxidizes.
The melting temperature of silica is about 1700° C. and its boiling temperature is 2700° C. At temperatures higher than 2000° C., silica is in liquid form. Numerous studies have shown that the formation of the initial layer of SiO2 takes place very quickly (quasi-instantaneous nucleation). In addition, the oxidation reaction gives rise to a large increase in the volume of the material associated with the variation in the molar volume of one mole of SiO2 compared with one mole of SiC. Furthermore, its coefficient of thermal expansion is small, thereby providing good thermal compatibility with the other refractory oxide layers that are present having coefficients of thermal expansion that are often much higher than that of the composite material. This significant increase in volume and the low permeability of oxygen in silica explain the protective nature of SiO2, which constitutes an effective barrier against diffusion of oxygen. This constitutes a particular example of passive oxidation.
Among the various systems that are fabricated by mixing (Zr/Hf)B2 and SiC, the system comprising 20% by volume of SiC (i.e. a (Zr or Hf)/Si atomic ratio equal to 2.7) presents a good compromise between adhesion to the composite material containing carbon and resistance to oxidation. Adhesion is enhanced by chemical and thermomechanical compatibility between the composite material and its coating. The low coefficient of thermal expansion of SiC is close to that of carbon. Adding SiC thus makes it possible to improve thermomechanical compatibility and thus avoid microcracks appearing. Nevertheless, under a wet or dry oxidizing atmosphere and/or at high temperature, silica evaporates and growth of this passive layer becomes very limited. Thus, at low pressure, it is possible for a transition to occur from passive oxidation to active oxidation of the SiC.
At a temperature higher than 2000° C., effective protection of such systems is weakened because of the active oxidation of silicon carbide producing gaseous SiO and leading to reopening of the pores in the refractory oxide skeleton containing at least HfO2 or ZrO2.
There exists a need to protect parts that are used in an oxidizing medium at temperatures higher than 2000° C.
This applies in particular to rocket engine components or to components of aeroengines of the turbojet type in which the steam and the carbon dioxide that are produced and ejected through the nozzle create an environment that is wet and oxidizing. This protection problem also arises for vehicle heat shields for re-entry into the atmosphere.